Magnetic dominance of axion electrodynamics: photon capture effect and anisotropy of Coulomb potential

Abstract

For magnetic fields larger than the characteristic scale linked to axion-electrodynamics, quantum vacuum fluctuations due to axion-like fields can dominate over those associated with the electron-positron fields. This conjecture is explored by investigating both the axion-modified photon capture by a strong magnetic field and the Coulomb potential of a static pointlike charge. We show that in magnetic fields characteristic of neutron stars $$\sim 10^{13}$$ ∼ 10 13 –$$10^{15}\;\mathrm{G}$$ 10 15 G , the capture of gamma photons prior to the production of a pair can prevent the existence of an electron-positron plasma, essential for explaining the pulsar radiation mechanism. This incompatibility is used to limit the axion parameter space. Our bounds improve existing outcomes in the region of mass $$m\sim 10^{-10}$$ m ∼ 10 - 10 –$$10^{-5}\;{\mathrm{eV}}$$ 10 - 5 eV . The effect of capture, known in QED as relating to gamma-quanta, is extended in axion electrodynamics to include X-ray photons with the result that a specially polarized part of the heat radiation from the surface is canalized along the magnetic field. Besides, we find that in the regime in which the dominance takes place, the running QED coupling depends on the field strength and the modified Coulomb potential is of Yukawa-type in the direction perpendicular to the magnetic field at distances much smaller than the axion Compton wavelength, while along the field it follows approximately the Coulomb law at any length scale. Despite the Coulomb singularity manifested in the latter case, we argue that the ground-state energy of a non-relativistic hydrogen atom placed in a strong magnetic field turns out to be bounded due to the nonrenormalizable feature of axion-electrodynamics.

Axion Electrodynamics in Strong Magnetic Backgrounds

“…Indeed, under such a restriction, radiation channels such as the recombination of pairs, cyclotron radiation and the photon splitting effect along with the main pair production mechanisms in highly-magnetized pulsars would be suppressed. This sort of quantum triviality [35][36][37] persists in domains other than the one discussed before [31].…”

“…q 2 0.e − q 2 < 4m 2 e . While the former relation introduces the condition B c ≪ B 0 along with b ≫ 1, the latter provides a characteristic value for q ⊥ [31]:…”

“…As a consequence, the following expression for the axion-modified Coulomb potential was found [for details we refer the reader to Ref. [31]]:…”

“…Theoretically, the problem linked to field strengths overpassing the corresponding electric E c = m/g and magnetic B c = m/g scales of AED have not yet attracted much attention, save in [30,31], where their respective relevance has been analyzed in connection with phenomenologies predicted to take place in black holes and neutron stars. While an electric background E E c induces an axionic instability, for magnetic fields B B c the theory is stable and -as in QED -the screening and refractive properties induced by quantum vacuum fluctuations of axion-like fields are predicted to be more pronounced.…”